My father was in the Royal Engineers and after WWII he was part of the unit dumping this stuff. He claimed there were also tonnes of mustard gas and very dodgy dynamite dumped too. The dynamite was sweating and gave them all headaches. Terrible job dumping this stuff in the Irish Sea off of a heaving ship.
My father used dynamite in his work (geological exploration). He was susceptible to migraine headaches.
I could always tell when he'd been personally setting charges, because he'd come home with a killer headache on those days. Not because of overage 'sweating' dynamite, but because of his high sensitivity to headache triggers like minute amounts of nitroglycerine vapor.
The funny thing about Beaufort's Dyke is that it's a possible route for a UK-Ireland tunnel (similar to the Channel Tunnel), although of course not now since it's full of unexploded ordinance.